Different Endings

Each character has 3 endings, here's how to obtain them:
ENDING 1: Do not ultra the 6th or 7th opponent. (Good side ending)
ENDING 2: Ultra the 6th, but not the 7th opponent. (Evil side)
ENDING 3: Ultra both the 6th and 7th opponent. (Neutral).

Fight Against Shadow Jago

First you need all 3 Endings of one character unlocked. (Check under Fight Archive> Cinematics)
Then you must Beat Arcade mode on Medium Difficulty or higher using the character who's endings are unlocked.
You must not loose any match, and you must perform an Ultra on Enemy #6 (Rival).
If done successfully, You will skip fight 7 (Fulgore) and go to Secret Fight 8 Instead.

Hidden remixed classic themes

If you start a match and have both players do nothing, after around 10 seconds, a remixed version one of the character's classic themes will play. The theme that plays corresponds with what character's stage you are on. Doing any action will cause the music to fade back into the regular theme.